204 



METAZOAN PHYLA 



small and short spines; they are found on sandy beaches, burying them- 

 selves just under the surface of the sand as the tide goes out but moving 

 about on the sand after the tide has returned. 



237. Holothurioidea. — The sea cucumbers, which make up this class, 

 differ from other echinoderms in the fact that they are greatly elongated 

 along the oral-aboral axis, the mouth being at one end and surrounded 

 by branched tentacles, while the anal opening is at the opposite end of 

 the body (Fig. 112). The body wall is muscular and possesses few and 

 small calcareous plates. The madreporite is internal. Tube feet are 

 present and serve as organs for clinging and for locomotion. 



Ari's-f-o-f-fe 



Ampu/fa/ 



Go/7ac^ 



Fig. 111. — Internal structure of a sea urchin. {From Dclage and Herouard, "Traite de 

 Zoologie Concrete" after Milne Edwards.) The oral wall of the shell has been removed and 

 the contents of the body are viewed from the oral pole with the Aristotle's lantern and 

 esophagus turned to the left. 



One type of sea cucumber is represented by those which conceal 

 themselves in the crevices between rocks and which have the tube feet 

 all around the body in five double rows. Some of the tube feet adjacent 

 to the mouth, as well as the tentacles, are used in procuring food. A 

 cloaca is present in a typical sea cucumber and contains the openings of 

 two long branched tubes, the respiratory trees (Fig. 113). Respiration 

 occurs in these as well as through the cloacal wall and the walls of the 

 tentacles and tube feet. The respiratory trees also serve as excretory 

 organs. The madreporite takes water in from the coelomic cavity. 

 Other sea cucumbers possess tube feet on only one side of the body and 

 travel about on that side, looking like huge caterpillars. Still others 

 burrow in the mud like earthworms and have no tube feet at all; they 



